High altitude, long-duration solar powered aircraft have been proposed for both commercial and military applications. For example, lighter-than-air aircraft have been proposed for cellular telephone applications. Military applications also include telecommunication applications as well as providing surveillance.
There is a domain in the upper stratosphere at 60,000 feet where it is ideal to position a lighter-than-air aircraft. This altitude allows on-board sensors to see over the horizon at least 350 miles in any direction. In most such applications, long duration station keeping is essential. Consequently, the issue is not in getting an aircraft to 60,000 feet, but in maintaining power so that the on-board sensors and electronics are continuously powered for extended periods of time, which may be from a few weeks to several months to even longer.
Electrical energy generated using solar cells or photovoltaic cells are typically used to power lighter-than-air aircraft. For example, U.S. Patent Application No. 2002/0005457 discloses a lighter-than-air aircraft powered with flexible solar cells integrated within the material covering the aircraft. Although the energy provided by solar cells is adequate to power lighter-than-air aircraft while in the sunlight, the challenge is to repeatedly get through the night. To keep a large lighter-than-air aircraft in a general location at 60,000 feet requires a significant amount of power. The solar panels not only need to take in enough solar energy to power the aircraft during the day, but also needs to take in additional power to be stored in batteries so that it can be used during the night.
In addition, extra power is needed to maintain position due to the upper winds or air currents at 60,000 feet, and for maintaining direction of the solar panels toward the sun as the direction of the sun changes throughout the day. This puts a bigger demand on the ability to store power for use during the night. One approach is to place more solar panels on the aircraft for collecting and storing the additional power, but this results in an increase of the weight of the aircraft. The greater the weight, the greater the volume of lift gas required, which increases the amount of material necessary to contain the lift gas. These increases in weight and volume impose additional power requirements.
As an alternative to placing more solar panels on the aircraft, one approach is to maintain an optimum position of the solar cells in relationship to the sun. For example, most all spacecraft are solar powered. In such spacecraft, the solar panels are rotatable so that an optimum angle can be maintained between the solar panels and the sun. However, these systems are not particularly advantageous on a lighter-than-air aircraft. In U.S. Pat. No. 6,371,409, solar panels mounted on an outer surface of a lighter-than-air aircraft are movable over a portion of the surface thereof to adjust for changes in the direction of the sun, or if maintained in a stationary position, for the inclination of the sun throughout the day.
Another approach to providing the power needed throughout the night is to use fuel cells. For example, the power requirements for the high altitude airship (HAA) as designed by Lockheed Martin Corp. are met by a combination of solar cells, fuel cells and batteries, wherein the fuel cells provide electrical power during the night. The fuel cells receive the gaseous elements of hydrogen and oxygen for generating electrical power.